1. Field of Invention
This invention relates to the design and construction of bridges, specifically to bridges with prefabricated deck units.
2. Prior Art
Cast-in-place concrete decks are the most commonly used type of bridge deck. However, the cast-in-place construction of concrete decks requires the placement of formwork, field placement of reinforcement, and field placement of concrete. This is a time consuming process and requires intensive field labor.
Full-depth precast concrete deck units that are used for bridge decks have been developed to overcome the disadvantages of cast-in-place concrete decks as listed above. Use of such deck units allows for the deck concrete and reinforcement to be placed in a controlled environment, improving the quality of the deck. Since the units are prefabricated, they can be delivered to a site and erected quickly.
Bridges using full-depth precast concrete deck units typically consist of a plurality of longitudinally spaced concrete units supported by longitudinal load-carrying members. These members are usually a single girder or multiple girders or beams. This member or members can be comprised of various materials including steel, concrete or composite material.
When no longitudinal post-tensioning is used in conjunction with a precast concrete deck, the use of cast-in-place joints between precast deck units is required so that reinforcement present the deck units can be lapped, whereby providing continuity at the joints. The cast-in-place joint requires extensive fieldwork and the uncompressed joint typically exhibits long term maintenance and durability problems.
An improvement that has been made to precast concrete decks is to introduce longitudinal post-tensioning. The post-tensioning can provide a compression force across the deck joints, whereby improving the durability of cast-in-place joints. The introduction of longitudinal post-tensioning also facilitates the use of match-cast joints in conjunction with precast concrete decks. However, all current precast bridge deck construction employs internal post-tensioning, wherein post-tensioning ducts or sheaths are embedded inside the concrete deck. The current practice of using internal post-tensioning has several disadvantages, including:
(a) the extensive ductwork in the precast concrete deck units requires the ducts to be placed very accurately so that they will align with the ducts in the adjacent unit;
(b) duct coupling is required at the joints between the precast concrete deck units. If a duct is not coupled properly, jointing materials can leak into the duct and cause duct blockage. This can result in significant construction delays and quality of construction problems;
(c) the internal post-tensioning is vulnerable to corrosion, particularly in climates where deicing chemicals are used. These chemicals can penetrate through the concrete and corrode the post-tensioning steel, especially at locations where the post-tensioning ducts are coupled;
(d) the longitudinal post-tensioning used in the precast concrete deck units, as used in current practice, only provides compression to the precast concrete deck units without contributing to the load-carrying capacity of the longitudinal load-carrying member or members on which the precast concrete units rest;
(e) internal post-tensioning is very difficult to inspect and often requires an indirect inspection method such as non-destructive testing (NDT);
(f) internal post-tensioning cannot be replaced in the event of corrosion. Therefore, the only option is to replace the entire deck system, which can result in significant construction and user delay cost;